The invention generally relates to electronic components, primarily electronic components which are included in electronic integrated circuits or are produced using the corresponding processing methods, and in particular to electric resistors of polycrystalline silicon, germanium or silicon-germanium, and to a method of manufacturing such resistors.
Resistors of polycrystalline silicon, also called polysilicon, have been used in the electronic circuit field for about thirty years. Methods of manufacturing polycrystalline silicon are well-known, as are methods of manufacturing resistors from polycrystalline silicon. It is also previously known how it is possible, by adding dopants, to control the resistivity of the polysilicon to a desired value. The general technology is described in the book xe2x80x9cPoly Silicon for Integrated Circuit Applicationsxe2x80x9d by T. Kamins, ISBN 0-89838-259-9, Kluver Academic Publishers, 1988.
In analog electronic circuits the requirements of stability of included resistors are extremely high: both the specifications in regard of largest allowable change of the absolute value of the resistance must be fulfilled, and possible changes of the resistances of resistors, which are matched to each other, must be such that the mutual relationship of the resistances of the resistors is all the time accurately maintained. This can only be done if the resistors are sufficiently stable during all of the time period when the circuit is used, i.e. if the resistances of the resistors maintain a sufficient constancy during all of this time period.
Previously known methods for influencing and particularly improving the long term stability of polysilicon resistors are described in the following documents: M. Rydberg and U. Smith, xe2x80x9cElectrical Properties of Compensation-Doped Polysilicon Resistorsxe2x80x9d, May 1998, sent to IEEE Trans. Electron Devices, M. Rydberg and U. Smith, xe2x80x9cThe Effect of Fluorine on the Electrical Properties of Polysilicon IC-Resistorsxe2x80x9d, May 1998, sent to IEEE Trans. Electron Devices, M. Rydberg and U. Smith, xe2x80x9cImprovement of the long-term stability of polysilicon integrated circuit resistors by fluorine dopingxe2x80x9d, Mat. Res. Symp. Proc. Vol. 472, (1997), M. Rydberg, U. Smith, A. Sxc3x6derbxc3xa4rg and H. Hansson, xe2x80x9cCompensation doping of polysilicon films for stable integrated circuit resistorsxe2x80x9d, Diffusion-and-Defect-Data-Part-B-(Solid-State-Phenomena), Vol. 51-52, pp. 561-566 (1996), the published International patent application WO 97/49103 which has inventors U. Smith and M. Rydberg and discloses a polysilicon resistor having a high long term stability resulting from a high doping with fluorine, and finally the published International patent application WO 97/10606 which has inventors U. Smith, M. Rydberg and H. Hansson and discloses a polysilicon resistor having an increased stability of its resistance because of such a high concentration of donors that charge carrier traps are blocked at the grain boundaries.
In U.S. Pat. No. 5,212,108 for M. S. Liu, G. A. Shaw and J. Yue, xe2x80x9cFabrication of stabilized polysilicon resistors for SEU controlxe2x80x9d stabilisation of resistors between and within different manufacturing batches is described. This patent is thus concerned with the statistical spread of resistance values in the manufacturing process whereas the problem discussed herein relates to the change of the resistance values in time.
In applications in which polysilicon resistors are used in critical portions of electronic circuits, the insufficient stability of the resistors is a known practical problem. The fact is that the resistors can, when being used, in an unforeseeable way change their resistance values. Such deviations from the value set by the designer, as well as deviations between the resistance values of resistors matched to each other, can jeopardize the operation of the electronic circuit in which such resistors are included. The cause of the instability is to search for in the unsaturated bonds existing in the grain boundaries in the material. The unsaturated bonds are formed in the boundaries between the individual monocrystalline grains in the polycrystalline material due to the fact that the periodic ordering of the silicon atoms in the shape of a crystal lattice does not exist there. The outermost silicon atoms in a monocrystalline grain therefore have not sufficiently many silicon atoms as close neighbours in order to be capable of forming the four bonds which are characteristic of the lattice of silicon crystals. The resulting unsaturated bonds act as traps for charge carriers and thereby bind charges to the grain boundaries what influences the capability of the material to transport charge carriers and thereby the resistivity of the material.
If the number of bonded charges would remain constant during the manufacture of the resistors and during all of the time when the resistor is used, no problems in regard of the stability of the resistors would exist. However, the number of traps can decrease if individual atoms can migrate to the grain boundaries and be attached to the unsaturated bonds and thereby prevent them from continuing to work as traps for the charge carriers. In the same way the number of traps can increase in the case where the atoms leave their positions at the grain boundaries and then each one leave a remaining unsaturated bond.
It is known that the unsaturated bonds can be blocked by hydrogen atoms in the grain boundary. Hydrogen can exist in a high concentration in layers deposited on a circuit containing a polysilicon resistor, for example in passivation layers of silicon dioxide and/or silicon nitride, what results from the special production thereof. The hydrogen atoms react with the unsaturated bonds in the polysilicon resistor and block them so that they cannot continue to work as traps. However, a problem associated with hydrogen atoms which have been bonded to the unsaturated bonds is that the bonding strength between hydrogen and silicon is low compared to for example the mutual bond between silicon atoms. The bonds between silicon and hydrogen can therefore be easily broken whereby the unsaturated bonds are again exposed. Since unsaturated bonds capture charge carriers this will result in that the value of the resistivity is changed. To the extent that the causes of the bonds being broken are known, they can be referred to a general increase of the temperature or to local temperature variations caused by an increased power production in critical points in the resistor. However, it can not be excluded that the bonds can also be broken because of kinetic or quantum-mechanical effects caused by the transport of charge carriers through the resistor.
Though the capability of the hydrogen atoms to block unsaturated bonds is what is primarily discussed in literature, it cannot be excluded that other atoms which happen to be placed in a grain boundary or leaves it in the manufacturing process and in the use of the resistor cause similar effects, if they have not the capability of being sufficiently strongly bonded to the silicon atoms of the grain boundary. Without indicating here the magnitude of the influence, it can be mentioned that it is also possible that dopant atoms which when using the resistor interact with the grain boundaries in a dynamic way can have the same influence on the resistivity as the hydrogen atoms. In the same way it cannot be excluded that also other kinds of atoms and unintentionally added impurities included in the resistor and/or in the circuit plate, of which the resistor normally is a part, can have the same influence.
It is an object of the invention to provide polysilicon resistors having a good long term stability, i.e. having a good constancy of their resistances, which in. a safe way can be used particularly in sensitive electronic circuits such as circuits of analog type, intended for example for measurements or intended to be part of sensors, in which the resistance values of the resistors for example included in amplifier circuits directly influence an output signal representing a measured value.
The solution of the problem presented above associated with a in some cases lacking or insufficient stability of polysilicon resistors is to arrange one or more stabilising layers or blocking layers at the very part of the resistors, which defines the resistances thereof. It has appeared that a stabilisation is obtained if it is ensured both that the polysilicon resistor is protected by blocking layers or diffusion preventing layers, in particular one or more oxide based blocking layer produced from transition metals having suitable thicknesses, which are capable of preventing movable kinds of atoms, such as for example hydrogen, from reaching the unsaturated bonds in the polysilicon, and that the added material does not influence and remove the effect of possible other optimizing treatments of the polysilicon resistors, for example such treatments which in themselves result in an increased stability, and that the added blocking layer is compatible with the remaining manufacturing process of the electronic circuit. The blocking layers can be located between the resistor and other layers in the complete resistor structure, such as between the resistor body and passivation layers of typically oxide or nitride or between the resistor body and other layers containing oxides or nitrides, particularly between the resistor body and such layers, which owing to their method of production contain hydrogen atoms. The blocking layers can be located on either side of the usually plate-shaped resistor or on both sides thereof and thus enclose it. The resistor can have a resistance defining part, the resistor part, and connection regions, which together with the resistor part forms the resistor body.
The transition metals preferably include titanium and tungsten. The fact that the diffusion of hydrogen through a TiO2-layer is restricted is described in Su-II Pyun and Young-Ci-Yoon, xe2x80x9cHydrogen permeation through PECVD-TiO/sub 2/film/Pd bilayer by AC-impedance and modulation methodxe2x80x9d, Advances in Inorganic Films and Coatings, Proceedings of Topical Symposium 1 on Advances in Inorganic Films and Coatings of the 8th CIMTEC-World Ceramics Congress and Forum on New Materials. TECHNA, Faenza, Italy, 1995, pp. 485-96. The fact that layers which contain titanium and tungsten, such as a Ti30W70-film, see the discussion hereinafter, can be given improved diffusion blocking properties owing to among other things oxide layers at their surfaces is disclosed in R. S. Nowicki et al., xe2x80x9cStudies of the Ti-W/Au Metallization on Aluminiumxe2x80x9d, Thin Solid Films, 53 (1978) pp. 195-205, and S. Berger et al., xe2x80x9cOn the microstructure, composition and electrical properties of Al/TiW/poly-Si systemxe2x80x9d, Applied Surface Science 48/49 (1991), pp. 281-287. In U.S. Pat. No. 5,674,759, xe2x80x9cMethod for manufacturing semiconductor device for enhancing hydrogenation effectxe2x80x9d, for Byung-Hoo Jung a method of manufacture of TFTs and MOSFETs is disclosed. The diffusion of hydrogen out of a plasma-nitride layer is prevented if on top of the nitride layer a layer of xe2x80x9ca material having a low hydrogen diffusion coefficient, or a refractory metalxe2x80x9d is applied. An overview of the oxides of the transition metals and their interaction with hydrogen atoms can in addition be found in C. G. Granqvist, xe2x80x9cHandbook of Inorganic Electrochromic Materialsxe2x80x9d, Elsevier 1995.
Though, in the present state of the art, it is not possible to demonstrate by an analysis, there are reasons to suppose that the oxide based blocking layers obtain their stabilising properties owing to the fact that they have an unordered structure including defects in the structure, which have a capability of binding hydrogen atoms and/or impeding the movements thereof through the layer.